Steel pipes used for pipe lines that transport natural gas or petroleum have been strengthened in order to improve transport efficiency by increase in operating pressure. In pipe lines laid in seismic areas and frozen ground areas, large plastic deformation may be caused by external force due to ground deformation. In pipe lines with plastic deformation having occurred therein, ductile fracture may occur by ductile cracks being generated and progressing from defects latent in parts of circumferential welded portions and the like or by necking occurring in base material portions.
Conventionally, in designing a welded joint, in order to prevent deformation and strain from centralizing in a welded joint portion from the viewpoint of prevention of brittle fracture, based on making higher (overmatching) the yield strength and tensile strength of a weld metal than those of the base material, overmatching of about 110% to 120% as compared to the base material has been demanded (see the “Embodiments of Invention” section of Patent Literature 1). However, recently, steels having sufficient brittle fracture characteristics are used for pipe lines used in regions where large ground deformation occurs, and ductile fracture is the form of fracture that has been problematic. Nevertheless, when a weld metal is selected, regardless of the form of fracture, joint designing is actually done such that overmatching of 110% or greater as compared to the base material yield strength and base material tensile strength is achieved.